Variable capacity vane pumps are well known and feature a capacity adjusting element in the form of a pump displacement ring, or slide, that can be moved to alter the eccentricity of the pump and hence alter the volumetric capacity of the pump. Typically, the ring is mounted within the pump body by a pivot pin and an appropriate control system, often a piston or pressurized chamber acting against a spring, is provided to move the ring about the pivot to obtain the desired equilibrium pressure from the pump.
While such pumps operate well, they do suffer from disadvantages in that the control system components tend to be relatively large as they must counter the imbalance of forces acting on the ring when moving the ring to alter the volumetric capacity of the pump. Specifically, the pressurized working fluid produced by the pump acts against the ring to force the ring in one direction. In order to act against this force, the control system for the ring typically must have larger components than would otherwise be necessary to move the ring. In many circumstances, especially in an automotive engine environment, these larger components require space which may not be available, or which could be put to better use.